The production of molded articles having an integral skin by the isocyanate-polyaddition process is known. It is carried out, for example, by introducing a reactive and, optionally, foamable mixture based on compounds containing several reactive hydrogen atoms and polyisocyanates into a mold (cf. for example British Pat. No. 969,114). The compounds containing reactive hydrogen atoms used in the above reference are preferably polyethers and polyesters containing hydroxyl groups. The polyisocyanates used are, for example, 2,4- and 2,6-tolylene diisocyanate and isomer mixtures thereof. Also suitable are the polyphenyl-polymethylene-polyisocyanates obtained by condensing aniline with formaldehyde, followed by phosgenation. Water and/or fluorochlorinated hydrocarbons, for example, may be used as blowing agents. Catalysts known in the art for the production of polyurethanes are also generally used.
By suitably selecting the starting components, it is possible, by this process, to produce elastic products, rigid products and also variants falling between these two extremes.
As explained in British Pat. No. 1,534,258, it is possible to use systems containing diamines as chain extending agents. Generally, however, a one-shot process is not feasible where diamine chain extenders are used. It is necessary first to prepare a "prepolymer" containing NCO groups. This "prepolymer" is then reacted with the diamine in a second stage to form the high molecular weight elastomer (U.S. Pat. No. 3,428,610).
The production of heavily stressed moldings generally requires the use of starting materials having a slightly branched structure which, after processing, give materials having a property spectrum resembling that of elastomers. Such moldings have been commercially produced for some time (for example, as soles in the shoe industry). Large moldings of this type are used in the automotive industry.
The starting materials are processed into automotive body components primarily by the so-called "reaction injection molding" (RIM) process. This process is a filling technique in which the highly active, liquid starting components are rapidly injected into the mold through high output, high pressure metering units after mixing in so-called "static impingement" mixing heads.
A detailed description of the reaction injection molding (RIM) process may be found, for example, in the following publications: Piechota/Rohr: "Integralschaumstoffe (Integral Foams)", Carl-Hanser-Verlag, Munich/Vienna 1975; Prepelka/Wharton: "Reaction Injection Molding in the Automotive Industry", Journal of Cell. Plastics, Volume II, No. 2, 1975; Knipp: "Plastics for Automobile Safety Bumpers", Journal of Cell. Plastics, No. 2, 1973.
It is possible by the reaction injection molding (RIM) technique to produce large moldings weighing up to 10 kg and more. These large moldings are used, for example, in the form of flexible automotive body components in the automotive industry. They are known in the automotive industry as so-called "soft face" elements, i.e., reversibly deformable front or rear parts of automobiles.
Reaction injection molding (RIM) has led to molding of parts, as described above, having a major advantage over the prior art. Namely, large quantities of two liquid, highly reactive starting products are rapidly delivered (in from about two to four seconds) and, at the same time, mixed and introduced into a mold where hardening to form the final molding takes place very quickly (from 15 sec. to 2 minutes).
Before this new technology could be adopted for practical use, the following three problems had to be solved.
First, because of the high reactivity of the two starting components (polyisocyanate and polyisocyanate-reactive compounds), the reaction mixture has to be introduced into the mold in the shortest possible time, a time which at most may be as long as the gel time. Thus, there was a need to develop high performance axial and radial piston pumps which, when built into high pressure machines, would enable throughputs of from 0.5 to 6.5 kg/second to be obtained. Such machines are described, for example, in German Offenlegungsschriften 1,778,060 and 2,146,054 (British 1,382,741).
Secondly, the exact metering of the two components in a preset ratio, depending on the particular formulation, throughout the entire duration of the "shooting-in" phase is necessary. Additionally, thorough admixture thereof from the first to the last drop is essential to obtaining a fault-free molding. Satisfactory admixture is made very difficult due to the high flow velocities of the two components and the extremely short residence time in the mixing chamber of the mixing head. This problem was solved by using so-called "static impingement" mixing heads operating on the "countercurrent injection principle" (cf. German Auslegeschrift 1,948,999 [U.S. Pat. No. 3,709,640] and also German Offenlegungsschriften 2,007,935 [U.S. Pat. No. 3,706,515]; 2,219,389 [U.S. Pat. No. 3,857,550]; and 2,364,501 [U.S. Pat. No. 3,926,219]).
Thirdly, when the reaction mixture enters the closed mold, the air contained therein is almost instantaneously displaced. In order to prevent undesirable inclusions of air and, therefore, faults in the end product, the liquid flowing in has to push the air along in front of it in the form of a "flow front" and force it out at predetermined slot-like vents. Thus, in order to completely prevent turbulence during filling, the material has to enter the mold over a considerable width in the form of a laminar flow along the mold wall. This problem has been overcome through the development of a certain gating technique using so-called "film gates" of the type described in German Offenlegungsschriften 2,348,658 (U.S. Pat. No. 3,991,147) and 2,348,608 (U.S. Pat. No. 3,908,966).
German Offenlegungsschrift 2,622,951 (British Pat. No. 1,534,258) describes how even highly reactive mixtures, i.e., one-shot mixtures, of active polyisocyanates, active aromatic polyamines, relatively high molecular weight polyhydroxyl compounds containing primary hydroxyl groups and strong catalysts, having gel times of less than one second, may be processed by this method. With such systems, the transition from the liquid to the solid phase is almost instantaneous, with the result that the liquid reaction mixture hardens on the walls of the mold.
It is possible by the instant process to fill voluminous and, at the same time, thin-walled (wall thickness &lt;3 mm), complicated mold cavities. Still-liquid material which continues to enter the mold under the filling pressure of the machine until the filling process is over would appear to force itself through between the peripheral zones of the molding hardened on the walls of the mold. This would appear to account for the fact that it is possible to produce moldings having greater weights than would appear theoretically possible by comparison of the gel time with the filling time (for a given filling volume per second). In addition, the reduced rate of viscosity increase of the instant invention over the prior art amine extended systems allows for longer flow times than were previously possible. On completion of the shot, the reaction mixture as a whole hardens so quickly that, in the case of highly reactive batches, the mold may be opened after less than thirty seconds and the molding removed therefrom.
These new systems retain the ability to produce a molding which has a low moisture absorbing potential as the previously known amine extended systems using the reaction injection molding (RIM) technique do. This is necessary in molding automotive parts for several reasons. It allows for easier finishing of the molded part and eliminates or greatly reduces blistering, bubbling, etc. of the finish applied to these molded parts. This low moisture absorptivity is also necessary to control part warpage and swelling. These necessary parameters are provided by the instant invention which, in addition, also provide longer gel times. Still another advantage of the instant invention is the greatly reduced tendency to form sink marks in the molded parts.
Although it is also possible in principle to fill voluminous mold cavities using the system according to German Offenlegungsschrift 2,622,951 (British Pat. No. 1,534,258), these systems still do not fully satisfy practical requirements. Because of the extremely short gel and filling times, these systems are still not optimally suitable for the production of very thin moldings. For example, the mass production of reversibly deformable front and rear sections of automobiles are not practical because faults attributable to incomplete filling of the mold are often encountered. This applies in particular where the reaction injection molding machines currently available are used. Although it would be possible to compensate for the above-mentioned disadvantages of these amine extended systems by using specialized reaction injection molding machines having a considerably increased output, this would require considerable additional capital investment in machinery. In addition, this would not alleviate the problems of large density gradients and/or sink marks common in the currently known reaction injection molding (RIM) amine extended systems.
An object of the instant invention is to improve the systems according to German Offenlegungsschrift 2,622,951 (British Pat. No. 1,534,258) in such a way that it is possible and practical to mass produce large volume, thin-walled moldings weighing up to 10 kg using available reaction injection molding machines. More particularly, this object is realized in the present invention by increasing the gel time and lowering the rate of viscosity increase of the amine extended systems referred to above without, at the same time, significantly affecting the short in-mold time required. The further objects of keeping moisture absorption low and reducing the tendency to form sink marks is realized in the instant invention through the particular polyol blends used. Another object of the instant invention is to improve the physical properties of the molded product (i.e. green strength, low temperature flexibility).
The achievement of the objects according to the present invention is surprising because, initially, it had been expected that the gel time would largely be determined by the more reactive amine component and not by the nature of the hydroxyl groups of the polyether. Particularly, it had not been expected that blends of two types of polyols would be superior to either type alone and that only those blends which contained a certain amount of polyoxyethylene (EO) groups would give an acceptable combination of properties.
The systems according to German Offenlegungsschrift 2,622,951 which, by virtue of the mechanical properties thereof, are ideally suitable for the production of large volume, flexible automotive body components ("soft-face" elements) have, for example, a gel time of approximately two seconds and lead to moldings which may be mold-released after from 1/2 to one minute. By using the polyether polyol blends of the present invention in otherwise the same formulation, it is possible to reduce the rate of viscosity increase and thereby substantially increase the flow time, without adversely affecting the green strength or water absorption characteristics of the molded part. The longer flow times of the formulations of the present invention allow for the production of larger volume moldings without the necessity for investing in higher throughput machinery.